The overall objective of this proposal is to demonstrate that chromosomal abnormalities are mechanistically related to rat mammary tumor progression. The carcinogen-induced rat mammary carcinoma system constitutes and excellent model for the study of breast cancer. In this system, activation of the rasH oncogene by chemical carcinogens was postulated to be the initiation event. Much less in know about further steps required to achieve malignancy. For instance, no chromosomal banding studies have been reported in this system. It is the main hypothesis of this proposal that gross chromosomal abnormalities play a fundamental role in mammary tumor progression. The specific goals of this proposal are: 10 to identify specific non-random chromosomal abnormalities in sequential studies of rat mammary tumors, comparing the results of two different protocols A) N-nitroso-N-methylurea (NMU) and B) 7,12-dimethylbenz (a)-anthracene (DMDA); 2) to monitor the cytogenetic progression of mammary tumors using a transplantation model, and correlating with different indicators of tumor progression; 30 to develop an in vitro system that will mimic the in vivo rat mammary carcinogenic events. Mimicing the intiation event the Ha-MSV v-rasH gene will be introduced into cultured rat mammary epithelial cells (RME) by a defective retroviral vector. To reproduce the malignant progression events chromosomal aneuploidy will be induced in RME cells using mitotic poisons such as Colcemid and diethlystilbestrol (DES). The phenotypic effects of both experimental interventions, i.e., v-rasH introduction and aneuploidy, will be analyzed when acting in combination and also separately. To monitor the phenotypic alterations RME cells will be subject to in vitro follow up and in vivo testing by inoculation in rat mammary fat pad. This in vitro approach will permit us to analyze and identify specific chromosomal abnormalities involved in immortalization and transformation and will complement the in vivo studies on rat mammary carcinogenesis.